The main plot is about a dog named Molly. It's a story that likely follows Molly's life experiences, her relationships with people and other animals, and her journey through different situations. The 'A Dog's Purpose' aspect might imply that Molly has some sort of mission or purpose in her life that she discovers as the story unfolds.
Well, 'Molly's Story' which is part of the 'A Dog's Purpose' series probably focuses on Molly. Maybe she goes through a series of adventures. She could be born in one place, perhaps a small town or a farm, and then through various events, she ends up in different homes. Along the way, she might meet some really kind people who love her and some not - so - nice ones too. And all these experiences contribute to her finding her purpose in life, which could be something as simple as bringing joy to her owners or something more profound like saving someone's life.